U.S. Pat. No. 6,319,451 discloses an injection molding method which comprises the steps of: supporting and positioning a core piece in a mold cavity by virtue of a plurality of positioning pins, injecting a molding material through a gate, retracting the positioning pins before the molding material contacts the positioning pins, and opening the mold and ejecting out a finished product after the molding material is cured and cooled.
Further, U.S. Pat. No. 6,033,611 discloses an injection molding method which comprises the steps of: supporting and positioning a core piece in a mold cavity by virtue of a plurality of positioning pins, injecting a first material through a gate, retracting partially the positioning pins before the first material contacts the positioning pins, curing the first material while the positioning pins are retracted to form an inner cover having a plurality of recesses which correspond to spaces occupied by the positioning pins in the mold cavity, injecting a second material through the gate so as to fill up the recesses and surroundingly cover the first material, and opening the mold after the second material is cured and cooled to remove a finished product having a double-layered structure.
However, because the positioning pins are retracted before the mold cavity is filled with the material, the core piece may be offset slightly, resulting in a reduction of positioning accuracy of the core piece. Therefore, there is still room for improvement.